Homologous recombination is a type of genetic recombination that occurs during meiosis the formation of egg and sperm cells. Homologous recombination dominates as the major form of dna repair in trypanosoma brucei, and is especially important for recombination of the subtelomeric variant surface glycoprotein during antigenic variation. Homologous recombination hr is essential for maintaining genome stability by promoting the accurate repair of dna lesions, such as doublestrand breaks dsbs and stalled replication forks bzymek et al. Dna replication and repair enzymes are present at the time of genetic recombination. Jun 23, 2019 the chapter on mutation and repair of dna dealt mainly with small changes in dna sequence, usually single base pairs, resulting from errors in replication or damage to dna. The initial steps of hr involve processing the dna ends of dsbs by exonucleases to generate 3 singlestranded dna ssdna tails figure 1.
The tumor suppressor activity of brca2 is attributed to its functions in genome maintenance. Two recombinases, rad51 and dmc1, then form nucleoprotein filaments on ssdna and search for dna homologous sequences leading to strand invasion dloops in in vitro assay. Reca is the most crucial component for the homologous also known as general. Non homologous end joining as a mechanism of dna repair deborah e. As part of a large scale analysis of proteinprotein interactions, known proteins implicated in replication, nucleotide excision repair, mismatch repair, base excision repair, non homologous end joining, homologous recombination and checkpoint pathways were used in yeast 2hybrid experiments to identify physical interactors in the predicted. A dramatic threat to the covalent structure of dna is posed by. Our gene editing approach is designed to work exclusively via homologous recombination, which has important advantages over the other dna repair pathway, non homologous end joining. Dna break repair by homologous recombination homologous. Rad51, palb2 and brca1, which also act as tumor suppressors. To maintain both viability and genome stability in response to such lesions cells have evolved two types of dsb repair pathways. In viruses, homologous recombination helps shape viral evolution.
Transcriptrnatemplated dna recombination and repair. Global crotonylome reveals cdylregulated rpa1 crotonylation. We hypothesized that the critical role of brca1 and brca2 in the repair of doublestrand breaks by homologous recombination hr was the underlying reason for this sensitivity. Recombination repair an overview sciencedirect topics. A central core of proteins, most critically the reca homolog rad51, catalyzes the key reactions that typify hr. Nonhomologous end joining as a mechanism of dna repair. The rad51 recombinase mediates the formation of dna joints that link homologous dna molecules. The two genes are part of a biochemical pathway that uses homologous recombination to repair dna double strand breaks homologous recombination repair, hrr. The deliberate targeted introduction of dna doublestrand breaks by endonucleases and their coordinated repair underpin key biological processes such as recombination between homologous chromosomes during meiosis or vdj rearrangements of immunoglobulin. Homologous recombination occurs in yeasts while in mammals, non homologous and joining dominates.
Homologous recombination also produces new combinations. Haber1 department of biology and rosenstiel basic medical sciences center, brandeis university, waltham, ma 024549110. In particular, we study brca2, a tumor suppressor involved in dna repair by homologous recombination. Depletion of usp9x leads to dna doublestrand breaks, some of which are triggered by replication fork collapse. Deficiency in either of the breast cancer susceptibility proteins brca1 or brca2 induces profound cellular sensitivity to the inhibition of polyadpribose polymerase parp activity. Much less is known about how damaged dna is repaired in mitochondria than about how it is repaired in the nucleus. Hsf2bp negatively regulates homologous recombination in. The chapter on mutation and repair of dna dealt mainly with small changes in dna sequence, usually single base pairs, resulting from errors in replication or damage to dna. Dna damage and repair summary department of molecular. Mechanism of homologous recombination nyu langone health.
Escherichia coli and shown to be involved in repair of dna damage 29, 74. Homologous recombination is a universal process, conserved from bacteriophage to human, which is important for the repair of doublestrand dna breaks. Dna repair, recombination and transposons flashcards quizlet. It is also important for producing genetic diversity in bacterial populations, although the process differs substantially from meiotic recombination, which repairs dna damages and brings about diversity in eukaryotic genomes. Homologous recombination hr is a dna metabolic process found in all forms of life that provides highfidelity, templatedependent repair or tolerance of complex dna damages including dna gaps, dna doublestranded breaks dsbs, and dna interstrand crosslinks icls.
As a result of this, the damaged dna can access to the homologous area of duplex dna and does a complementary pair of base pairing. Mechanism of homologous recombination and implications. Homologous recombination dna break repair by homologous recombination this requires specialized factors. Technical advances in protein tagging and microscopy are providing insight into the in vivo biochemistry and cellular organization of this process.
Each line represents a chromosome or segment of a chromosome. It is an essential repair process for dividing cells because a replication fork may arrive at a damaged site, such as a thymine dimer, before the excision repair system has eliminated damage. Homologous recombination in dna repair and dna damage. A model for donor dna processing in recd transconjugants and transductants was proposed. Homologous recombination repairs dna before the cell enters m phase of mitosis. Hsf2bp negatively regulates homologous recombination in dna. Rad50, a component of the mrn complex mre11, rad50, nbs1, is central to homologous recombination through facilitating resection and governing the dna damage response. Homologous recombination is a major dna repair process in bacteria. Double strand break repair homologous recombination youtube. Homologous recombination occurs naturally in eukaryotic organisms, bacteria, and certain viruses and is a powerful tool in genetic engineering. Noncanonical regulation of homologous recombination dna. Double strand break repair by homologus recombination youtube. Dna strands during homologous recombination gene conversion. Homologous recombination repair hrr is a complex templatedirected repair.
Feb 11, 2015 this dna repair animation explains double strand break repair including homologous recombination. These exonucleases were shown to be essential for cell viability, dna repair of uv and. Homologous recombination in humans, the central player of the hr process is rad51. Homologous genetic or dna recombination is an exchange or replacement of a segment of parental dna with a segment having the identical or very similar i. Mar 17, 2020 homologous recombination dominates as the major form of dna repair in trypanosoma brucei, and is especially important for recombination of the subtelomeric variant surface glycoprotein during antigenic variation. Homologous recombination can repair doublestrand breaks without any change in dna sequence, but nonhomologous end joining always involves a loss of genetic information because the ends are degraded by nucleases before they can be ligated back together. Homologous recombination an overview sciencedirect topics. It works during the most critical point of cell replicationafter dna is copied but before it divides. Homologous recombination hr comprises a series of interrelated pathways that function in the repair of dna doublestranded breaks dsbs and interstrand crosslinks icls. Two recombinases, rad51 and dmc1, catalyze the pairing and shuffling of homologous dna sequences in eukaryotic cells via a filamentous intermediate on ssdna called the presynaptic filament. In bacteria, homologous recombination is a major mechanism of dna repair and facilitates the incorporation into dna of genetic material received via horizontal gene transfer and transformation.
Homologous recombination hr repair deficiency predisposes to cancer development, but also sensitizes cancer cells to dna damageinducing therapeutics. In mitotic cells, heteroduplex dna formation between sister chromatids is essential for homologydirected repair hr of dna doublestrand breaks dsbs, stalled replication. Barnes in spite of its essential role as the carrier of genetic information, dna is not an inert structure. In all cells, genetic recombination is used to repair dna breaks and, as a result, genetic information is exchanged between homologous dna molecules. We monitored dna repair pathways in mds and their alterations during disease progression. Most of the extended dloops in somatic cells are disrupted and subsequently repaired by synthesisdependent strand annealing sdsa.
During meiosis i, hr also facilitates the formation of chiasmata, the cytological manifestation of genetic crossovers, thus ensuring correct chromosome. Precise gene replacement in rice by rna transcripttemplated. They result in genetic recombination which may lead to chromosomal translocation, broken chromosomes, and finally cell death. Intersection between homologous recombination, dna replication and dna repair, volume 601, the latest release in the methods in enzymology series, continues the legacy of this premier serial with quality chapters authored by leaders in the field homologous genetic recombination remains the most enigmatic process in dna metabolism. Mechanism of homologous recombination and implications for. A diagram of the strategy with notations as in figure 1a, except that the target dna was included in total, genomic dna purified from e. Feb 25, 2015 homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of dna. Recombination in mitochondrial dna mtdna was documented more than 4 decades ago, but the underlying molecular mechanism has remained elusive. Nhej is the predominant mechanism in mammalian cells, whereas homologous recombination depends on the availability of homologous sequences and is more frequent in drosophila preston et al. Pdf dna doublestrand break repair by homologous recombination.
Two major dna repair mechanisms, non homologous endjoining nhej and homologous recombination, are responsible for the repair of dsbs. Homologous recombination is responsible for highfidelity dna repair. For homologous or general recombination, each homologous chromosome is shown as a different shade of blue and. In addition, recombination provides critical support for dna replication in the recovery of stalled or broken replication forks, contributing to tolerance of dna damage. Homologous recombination hr proceeds as shown in the figure using a homologous donor dna. Dna repair pathways direct enzymatic repair base excision repair nucleotide excision repair mismatch repair doublestrand break repair non homologous end joining homologous recombination. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of dna, known as double strand. Homologous recombination, the exchange of genetic material between two strands of dna that contain long stretches of similar base sequences. Dna damage is a fact of life as a consequence of endogenous sources and processes as well as exogenous sources. Paired chromosomes from the male and female parent align so that similar dna sequences from the paired chromosomes cross over each other. Remarkably, proper function of the recombinases requires interactions with. The ubiquitinspecific protease 9x usp9x contributes to genome stability during dna replication and chromosome segregation. Recombination is the production of new dna molecules from two parental dna.
The high incidence of mutations and cytogenetic abnormalities in patients with myelodysplastic syndrome mds suggests that defects in dna repair mechanisms. Resynthesis of the damaged region is accomplished using the undamaged molecule as a template. Now it is known that some amount of dna synthesis also takes place during recombination. Homologous recombination and the repair of dna doublestrand. Figure 1 a doublestrand break repair model for homologous recombination. The genome is susceptible to potentially mutagenic threats of both endogenous and environmental origin. Homologous recombination is a molecular process that has multiple important roles in dna metabolism, both for dna repair and genetic variation in all forms of life 1. May 05, 20 nhej is especially important before the cell has replicated its dna, since there is no template available for repair by homologous recombination. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of dna, known as double strand breaks. Genetic recombination with diagram molecular biology. Mutations in this gene cause susceptibility to breast and ovarian cancer. Homologous recombination enables the cell to access and copy intact dna sequence information in trans, particularly to repair dna damage affecting both strands of the double helix. Jun 26, 2001 doublestrand break repair mechanisms rely on enzymes which evolved primarily to deal with developmentally programmed doublestrand breaks.
Cells repair dna via homologous recombination through the highly precise incorporation of correct dna sequences that are homologous, or matching, to the site of damage. Specifically, cdyl negatively regulated kcr of rpa1, and mutation of the kcr sites of rpa1 impaired its interaction with singlestranded dna andor with components of resection machinery, supporting a key role of rpa1 kcr in homologous recombination dna repair. Homologous recombination hr 2 is essential to access the redundancy of genetic information that exists in the form of sister chromatids or homologous chromosomes when both strands of the dna double helix are compromised. The most common form of hdr is homologous recombination. Homologous recombination in dna repair and dna damage tolerance. The dna sequence of a chromosome can change in large segments as well, by the processes of recombination and transposition. Postreplication kevin p rice,university of wisconsinmadison, madison, wisconsin, usa michael m cox,university of wisconsinmadison, madison, wisconsin, usa recombinational dna repair represents the primary function for homologous dna recombination in. Dna sequence information in trans, particularly to repair dna damage. Realtime analysis of doublestrand dna break repair by. Rrp6exosc10 is required for the repair of dna doublestrand. To cope with the large variety of dna damage, several different repair reactions have evolved, these include base excision repair, nucleotide excision repair.
Introduction homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of dna. Discontinuities in dna strands, specifically doublestrand dna breaks and singlestrand dna gaps, attract the enzymes responsible for the initiation of homologous recombination. Homologous recombination and the repair of dna double. Realtime analysis of doublestrand dna break repair by homologous recombination wade m. Double strand break repair homologous recombination. We are interested in dna repair and its link to tumorigenesis. Homologous recombination hr is a dna metabolic process found in all forms of life that provides highfidelity, templatedependent repair or tolerance of complex dna damages including dna gaps. A prerequisite for homologous recombination to occur is the availability of a wildtype dna template in the vicinity of the broken dna molecule, which is required for informational re. Other examples of homologydirected repair include singlestrand annealing and breakage. Rad50 promotes dna repair by homologous recombination and. The hdr mechanism can only be used by the cell when there is a homologous piece of dna present in the nucleus, mostly in g2 and s phase of the cell cycle. Deficiency in the repair of dna damage by homologous. Recombination repair is a mechanism for generating a functional dna molecule from two damaged molecules. Dna recombination and repair in cases where dna is severely damaged, a cell will engage in a phenomenon called the sos response in an effort to salvage a functioning set of genetic information.
This dna repair animation explains double strand break repair including homologous. Crossing over results in a shuffling of genetic material and is an important. Here we identify an hrdefect hrd gene signature, which can be used to functionally assess hr repair status without interrogating individual genetic alterations in cells. Mechanisms of dna recombination and genome rearrangements. This mechanism of repair only takes place only when 2 dna double stranded duplex contains extensive region of homology. Homologous recombination talking glossary of genetic. Dna doublestrand breaks dsbs are potentially lethal lesions if unrepaired, and their misrepair can give rise to chromosomal rearrangements, a hallmark of cancer cells 1,2. James haber on mechanisms of dna repair by recombination, part of a collection of online lectures. The end result of the repair by sdsa is always a noncrossover outcome. Homologous recombination repair intermediates promote.
Resolvase osgen1 mediates dna repair by homologous. Various models to explain homologous genetic recombination have been proposed based primarily on genetic observation in bacteria and fungi. Dna repair reactions, mutants of the corresponding genes are hypersensitive to dna damaging agents, especially those, such as ionizing radiation and figure 1 biological roles of hr. Important biological contexts of hr are dna replication support and the repair of dna doublestrand breaks dsbs in somatic cells and during. Dsbs can be repaired by homologous recombination or non homologous end joining. Pdf homologous recombination and the repair of dna. Dna replaced with same sequence homologous recombination target replaced with donor dna sequence. This process is essential for the repair of doublestranded dna breaks that occur during normal dna replication or by dna damaging agents. Homology directed repair hdr is a mechanism in cells to repair doublestrand dna lesions. Homologous recombination is a ubiquitous process that plays a key role in the repair of dna damage and in restarting replication forks that have stalled or aborted as a consequence of the fork encountering dna lesions see cox 1998. Differential expression of homologous recombination dna. Mar 10, 2017 homologous recombination repair is a dna repair process that includes the invasion of an undamaged dna molecule by a damaged molecule of identical or very similar sequence. Pdf mechanism of eukaryotic homologous recombination. Mar 18, 2019 one of the main obstacles to gene replacement in plants is efficient delivery of a donor repair template drt into the nucleus for homologydirected dna repair hdr of doublestranded dna breaks.
This dna repair animation explains double strand break repair including homologous recombination. Homologous recombination enables the cell to access and copy intact. In cases where dna is severely damaged, a cell will engage in a phenomenon called the sos response in an effort to salvage a functioning set of genetic information. Pdf homologous recombination hr is critical both for repairing dna lesions in mitosis and for chromosomal pairing and exchange during meiosis find. Homologous recombination as a mechanism for genome. These functions of brca2 depend on its association with other hr proteins. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of dna, known as doublestrand breaks. Homologous recombination repair genetics hrr genes mutations in brca1 and brca2 greatly increase the risk of breast, ovarian, and pancreatic cancers. Recent studies have revealed the presence of a rad52type recombination system.